One thing you can try, that's free, for OneNote. Turn on Airplane mode when you're in class/lecture. I've been doing this for years with OneNote and am convinced that a lot of the lagginess you see is due to the app trying to synchronize every single pencil stroke or letter you type, as you're performing that action.

If you turn on Airplane Mode (or at least turn off wifi) while you're actively adding new content, you should find a lot of the lagginess disappears. The iPad will run cooler, too. Then after you're done inputting new content, you can turn off Airplane Mode and let it sync.

Another free thing to try is print pdf's to a larger size paper than the original pdf and save that as a 2nd pdf. The way this works for me is, say I have letter-sized pdf. If you print that as a new pdf, but specify that you want, say, Ledger size, it SHOULD create a new pdf with your original content, shrunk down somewhat, on the left-hand side of the new page, with empty space to the right & below, up to the size of the new page.

YMMV.